Fuel cell systems mounted on fuel cell vehicles generate power by means of an electrochemical reaction, which is accompanied by the production of water. In general, the fuel cells of such a fuel cell system are configured as a fuel cell stack obtained by stacking a number of the smallest constitutional units called cells. In a case of a general polymer electrolyte fuel cell, as shown in FIG. 6, a cell 101 includes an anode 102 and a cathode 103 configured to supply hydrogen and air (oxygen), respectively, between which there are disposed diffusion layers 104 and 105, catalyst layers 106 and 107 for activating the reaction, and, in the center, an electrolyte membrane 108 configured to selectively transmit hydrogen ions.
Hydrogen molecules supplied to the anode 102 turn into active hydrogen atoms in the catalyst layer 106 present on the surface of the electrolyte membrane 108 on the anode 102 side, and further turn into hydrogen ions, releasing electrons. This reaction, which is illustrated as (1) in FIG. 6, is expressed in Formula 1 below.H2→2H++2e−  (Formula 1)
The hydrogen ions generated by Formula 1 transfer through the electrolyte membrane 108 from the anode 102 side to the cathode 103 side together with the moisture contained in the electrolyte membrane 108, while the electrons transfer to the cathode 103 through an external circuit 109. By this electron transfer, current flows in a load (e.g. a vehicle's traction motor) 110 arranged in the external circuit 109.
On the other hand, the oxygen molecules in the air supplied to the cathode 103 turn into oxygen ions in the catalyst layer 107 by receiving the electrons supplied from the external circuit 109, and then turn into water by bonding to the hydrogen ions transferring through the electrolyte membrane 108. This reaction, which is illustrated as (2) in FIG. 6, is expressed in Formula 2 below½O2+2H30+2e−→H2O  (Formula 2)
Part of the water thus produced transfers from the cathode 103 to the anode 102 due to concentration diffusion. In the chemical reactions described above, various kinds of loss occur inside the cell 101 such as resistance overvoltages attributable to the electrical resistances of the electrolyte membrane 108 and the electrodes, an activation overvoltage due to the electrochemical reaction between hydrogen and oxygen, and diffusion overvoltages due to the transfer of hydrogen and oxygen through the diffusion layers 104 and 105. The waste heat generated accordingly needs to be dissipated.
There are a water-cooled type and an air-cooled type for fuel cell systems including the above cell 101.
First, FIG. 7 shows the configuration of a general water-cooled fuel cell system of a conventional fuel cell vehicle. A fuel cell system 201 shown in FIG. 7 includes a fuel cell stack 202 obtained by stacking a number of cells, or the smallest constitutional units mentioned above, and also includes a hydrogen-gas supply device 203 configured to supply hydrogen gas to the fuel cell stack 202.
The hydrogen-gas supply device 203 introduces compressed hydrogen gas stored in a high-pressure hydrogen tank 204 into an anode intake part 207 of the fuel cell stack 202 by way of a hydrogen supply pipe 205 via a pressure-reducing valve 206. In this process, the temperature of the hydrogen gas drops due to adiabatic expansion of the gas, which in turn cools down hydrogen-related components including the hydrogen tank 204 and those between the hydrogen tank 204 and the fuel cell stack 202 such as the hydrogen supply pipe 205 as well as a shutoff valve and the pressure-reducing valve 206 given in an intermediate portion thereof.
On the other hand, the fuel cell system 201 includes an air supply duct 208 for supplying air to the fuel cell stack 202, and an air exhaust duct 209 for exhausting surplus air from the fuel cell stack 202. In the air supply duct 208, ambient air drawn through a filter 210 is compressed by a high-pressure compressor 211 and is then introduced into a cathode intake part 212 of the fuel cell stack 202. As a result, power generation is performed in the fuel cell stack 202.
The surplus air left unused in the power generation in the fuel cell stack 202 is exhausted to the air exhaust duct 209 through a cathode exhaust part 213 of the fuel cell stack 202 as cathode exhaust. The cathode exhaust exhausted to the air exhaust duct 209 is separated from part of the water in the exhaust by a steam separator 214, and then released to the atmosphere via a back pressure valve 215 aiming to control the pressure of the cathode system.
Meanwhile, the surplus hydrogen gas left unused in the power generation in the fuel cell stack 202 is exhausted to a hydrogen purge pipe 217 through an anode exhaust part 216 as anode exhaust. The hydrogen purge pipe 217 is connected to an intermediate portion of the air exhaust duct 209. Like the cathode exhaust, the anode exhaust exhausted to the hydrogen purge pipe 217 flows through a steam separator 218 as in the case of the cathode exhaust, and is then mixed to the cathode exhaust in the air exhaust duct 209 via a purge valve 219.
The amount of flow of the purged hydrogen exhaust, or the anode exhaust from the anode exhaust part 216, is smaller than that of the cathode exhaust to a large extent. Thus, the purged hydrogen from the anode exhaust part 216 can be released to the atmosphere at a concentration of 4%, which is the lower explosive limit, or lower with the help of the cathode exhaust. Note that in some fuel cell systems, the hydrogen purge pipe 217 is connected to the anode intake part 207 by a hydrogen return pipe 220, and a hydrogen pump 221 provided to the hydrogen return pipe 220 is used to re-circulate the anode exhaust to the anode intake part 207, for the purpose of improving the use efficiency of the hydrogen.
Now, a cooling system 222 of the water-cooled fuel cell system 201 will be described. The cooling system 222 includes a radiator 223 configured to cool down cooling water of the fuel cell stack 202. In the cooling system 222, a cooling loop is formed by connecting the fuel cell stack 202 to the radiator 223 by a cooling-water inlet passage 224 and connecting the radiator 223 to the fuel cell stack 202 by a cooling-water outlet passage 225.
The cooling system 222 includes a water pump 226 in the cooling-water inlet passage 224 connected to either an upstream or downstream side (downstream side in FIG. 7) of the fuel cell stack 202 to thereby pump the cooling water to the radiator 223. The cooling water having cooled down the fuel cell stack 202 exchanges its heat with the atmosphere in the radiator 223 and is then returned again to the fuel cell stack 202 through the cooling-water outlet passage 225.
This cooling system 222 is provided with a heating device 227. The heating device 227 includes a heating passage 228 connecting the cooling-water inlet passage 224 and the cooling-water outlet passage 225, and also includes a heater core 230 for warming up the cabin in the heating passage 228 with a regulation valve 229 therebetween in parallel with the radiator 223. When heating is needed, the heating device 227 supplies the cooling water, which is hot, to the heater core 228 by opening the regulation valve 229 and drives a fan 231 for blowing air, to thereby warm up the cabin.
As described above, the water-cooled fuel cell system 201 includes many accessories such as the compressor 211 for compressing the air introduced into the air supply duct 208, for the purpose of improving the output density of the fuel cell stack 202. Such a water-cooled fuel cell system 201 then results in a more complicated, larger, heavier, and more expensive system. In contrast, there is an air-cooled fuel cell system achieving simplification of the system by eliminating accessories such as the compressor and employing air cooling for the cooling of the fuel cells.
FIGS. 8 and 9 show an air-cooled fuel cell system 301. As shown in FIG. 8, like the above-described water-cooled fuel cell system 201, the air-cooled fuel cell system 301 includes a fuel cell stack 302 obtained by stacking a number of cells, or the smallest constitutional units, and also includes a hydrogen-gas supply device 303 configured to supply hydrogen gas to the fuel cell stack 302. The hydrogen-gas supply device 303 introduces compressed hydrogen gas stored in a high-pressure hydrogen tank 304 into an anode intake part 307 of the fuel cell stack 302 by way of a hydrogen supply pipe 305 via a pressure-reducing valve 306. In this process, the temperature drop of the hydrogen gas due to its adiabatic expansion cools down hydrogen-related components including the hydrogen tank 304, the hydrogen supply pipe 305, the pressure-reducing valve 306, and the like.
Here, in general, the air-cooled fuel cell system 301 does not include the high-pressure compressor on the cathode intake side, unlike the water-cooled fuel cell system. As shown in FIG. 9, the fuel cell system 301 includes an air supply duct 308 for supplying air to the fuel cell stack 302, and an air exhaust duct 309 for exhausting surplus air from the fuel cell stack 302. The air supply duct 308 supplies ambient air drawn through a filter 310 into a cathode intake part 312 of the fuel cell stack 302 by means of a low-pressure blower fan 311.
Moreover, the air supplied to the cathode intake part 312 is not only used as the reactant gas with the hydrogen for the power generation reaction in the many cells stacked in the fuel cell stack 302, but also functions as a cooling medium to remove the waste heat in the fuel cell stack 302 to cool down the fuel cell stack 302.
As shown in FIG. 9, the surplus air after the reaction with the hydrogen and the air having cooled down the fuel cell stack 302 are exhausted to the air exhaust duct 309 through a cathode exhaust part 313 of the fuel cell stack 302 as cathode exhaust, and then released to the atmosphere. The surplus hydrogen gas left unused in the power generation in the fuel cell stack 302 is exhausted to a hydrogen purge pipe 315 through an anode exhaust part 314 as anode exhaust. The hydrogen purge pipe 315 is connected to an intermediate portion of the air exhaust duct 309. The anode exhaust exhausted to the hydrogen purge pipe 315 is mixed to the cathode exhaust in the air exhaust duct 309 via a purge valve 316. When the hydrogen-gas purge on the anode side is performed, the exhausted hydrogen gas is diluted to its lower explosive limit or lower with the help of the cathode exhaust and is then released to the atmosphere.
The air-cooled fuel cell system 301 which uses the low-pressure blower fan 311 to supply air as both the reactant gas and the cooling medium as described above can achieve a reduced power consumption as well as a smaller, lighter, and simpler system. However, since the amount of air flow is limited, the cooling performance is lower than that of the water-cooled fuel cell system described above. For this reason, the operable temperature range of the fuel cell stack 302 is narrow in some cases, which possibly leads to overheating of the fuel cell stack 302 during a high-temperature period such as in the summer.
As described above, in the water-cooled fuel cell system and the air-cooled fuel cell system, when hydrogen, which is the fuel, is supplied to the fuel cell stack from the hydrogen tank storing the hydrogen in the form of high-pressure gas, the hydrogen gas, which is the fuel, is cooled down by its adiabatic expansion. The hydrogen gas thus lowered to a low temperature in turn excessively cools down the various hydrogen-related components including the hydrogen tank itself and also those provided between the hydrogen tank and the fuel cell stack such as the pressure-reducing valve and the regulator for the pressure regulation. The excessive cooling has been pointed out as possibly affecting the durability and reliability of these hydrogen-related components.
To avoid such inconvenience, Japanese Utility Model Registration Application Publication No. Hei 1-77267 and Japanese Patent Application Publication No 2007-161024, for example, disclose a technique in which a hydrogen-gas pipe and a cooling-water pipe for cooling down fuel cells are disposed adjacent to each other, or a hydrogen-gas pipe and a pipe for exhaust gas from a fuel cell system are disposed adjacent to each other. Moreover, Japanese Patent Application Publication No. 2005-44520, for example, discloses a technique in which a hydrogen-related component is arranged at such a position as to be capable of receiving the heat released from a radiator in a cooling system of a fuel cell stack.